If it's a critical situation, use a torgue wrench and consult the manual. Otherwise, my rule is 'wrist-tight', ie: thumb, forefinger and bird-finger cradling the head of the ratchet and twist as much as you can from there (if you have a normal sort of grip). There is no substitute for followng the manual, but wrist-tight is always safe.

What grade? How long? Why isn't this in mechanical engineering? Who's on first?

0.1% is the elastic limit for most steels --- once you've "set up" against the pieces you're holding together, 0.1% of the bolt length divided by the thread pitch in turns; depending on diameter, grade, finish, and lubrication, you'd best hit a handbook --- I ain't gonna type one. If you're playing with automotive head bolts, you'd best find out if someone stuck you with "torque to yield" specifications --- 1% elongation or more, resulting in permanent deformation --- USE ONCE ONLY; this is one of Detroit's cost cutting measures --- machine measures torque while tightening bolt and quits once it is constant vs. no. of turns completed --- torques to failure about 10% of the time, but, what the hell, that's good enough for the consumer market. The idea was that it results in equal tension on all head bolts --- great, if they're "dogboned" --- not so great if they aren't --- they fail at the thread root with a greater variation in tension than results from variations in thread friction using a torque wrench.

I quite agree thanks Bystander, i have all ways had respect for the nut, bolt,
thread, it is amazing how much variation there is in say a 10mm nut and bolt,
some times you can (rock) the nut on the bolt, other times the nut can be a
(tight) fit.

There is no goloden rule for torque. Technically, the torque is related back to the loading the joint must withstand. That load combines with the material of the fastener and the effective thread area. They are always different.

I have seen general torque recommendations given by fastener manufacturers, but even they say that it is not a steadfast rule. Analysis should always be done.

There is no substitute for followng the manual, but wrist-tight is always safe.

I just noticed that I used stupid phraseology there. I did not mean that it's safe as in "it won't come apart"; I meant that there's little chance of damaging the bolt or nut or stripping the threads of the component. Sorry if there was any misunderstanding.